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We study the vibrational density of states (VDOS) in the i-AlCuFe quasicrystal measured by different methods: (i) 57 Fe inelastic nuclear-resonant absorption (INA) of X-rays (ii) and time-of-flight inelastic neutron scattering (INS). The samples studied were i-Al 62 Cu 25.5 Fe 12.5 with different isotopic substitutions. INA measures the Fe-partial VDOS while INS the neutron-weighted ‘generalised’ GVDOS. We find the unexpected result that the iron-partial VDOS has a single sharp maximum. The neutron-weighted GVDOS for different isotopes (natural Cu and 65 Cu; natural Fe and 57 Fe shows that the copper-partial VDOS is strongly peaked as well but at significantly lower energy. These results are discussed in light of the icosahedral quasicrystal model of Katz and Gratias, as well as specific heat results which indicate a discrepancy between the speed of sound, the specific heat and the low energy limit of the VDOS.

We present an extensive triple-axis neutron-scattering study of phason hopping in a single-domain sample of a perfect icosahedral ${\mathrm{Al}}_{70.4}{\mathrm{Mn}}_{8.6}{\mathrm{Pd}}_{21.0}$ quasicrystal. The quasielastic intensities exhibit important anisotropies. They are compared with models at various levels of sophistication. This comparison strongly suggests the occurrence of simultaneous correlated jumps. The important correlations are directed along the threefold axes.

The rotational spectra of CH 4 at low temperatures have been used to investigate the disordering influence of a restricted geometry on the orientational structure of methane condensates in silicagels and Controlled Pore Glasses. A cross-over from complete orientational disorder to the partially ordered phase II of pure methane was observed with increasing pore diameter d at d ∼120 A.

Nous expliquons la difference entre la diffusion coherente et la diffusion incoherente des neutrons par une analogie avec l'experience de diffraction par une double fente de Young. Nous montrons que le langage adequat pour calculer les signaux coherents est base sur une description en termes de configurations dans lesquelles les particules identiques qui peuvent etre echangees sont considerees comme indiscernables. Nous illustrons notre demarche avec un exemple tire de la physique des quasicristaux.

Elastic and inelastic neutron scattering are used to study the structure and dynamics of single-wall carbon nanotubes (SWNT) self-assembled into nanobundles (NBSWNT). Suspensions of NBSWNT are characterized by small-angle neutron scattering. Neutron diffraction is used as a useful tool to study the structure of both the SWNT and NBSWNT. Calculations on finite-size bundles are compared to the data in order to estimate the distribution of tube diameters. Finally, we present time of flight inelastic scattering measurements of the phonon density of states and discuss the main features of the spectra in comparison with calculations.

Quantum ordering, mixing, and mobility of isotopic hydrogen mixtures adsorbed on graphite (0001) is studied in the monolayer range by diffraction, small angle (SANS), and quasielastic neutron scattering (QENS). Solid mixtures of ${\mathrm{H}}_{2}\ensuremath{-}{\mathrm{D}}_{2}$ commensurate with the graphite substrate remain random isotopic mixtures from 18 K down to 3 K. Incommensurate solid mixtures of ${\mathrm{H}}_{2}\ensuremath{-}{\mathrm{D}}_{2}$ at monolayer completion with and without a partial second layer adsorbed on top show a tendency towards phase separation and clustering as the temperature is reduced below 20 K. This tendency is monitored both by the change of the diffraction peak intensity with temperature and by features of the SANS measurements discussed in the text. Clustering slows down or stops at $\ensuremath{\approx}8 \mathrm{K}$ without reaching the full isotopic phase separation predicted on quantum-mechanical grounds. The QENS measurements measured the mobility of pure HD and $\mathrm{HD}\ensuremath{-}{\mathrm{D}}_{2}$ mixtures for two coverages. The mobility in the mixture is enhanced over the one of pure HD at the same coverage and temperature, an effect which can be related to a bond softening of the heavier ${\mathrm{D}}_{2}$ molecule due to quantum motion.

We have performed inelastic magnetic neutron scattering experiments on ${\mathrm{La}}_{2\ensuremath{-}x\ensuremath{-}y}{\mathrm{Sr}}_{x}{\mathrm{Nd}}_{y}{\mathrm{CuO}}_{4} (0l~xl~0.2$ and $0.1l~yl~0.6)$ in order to study the Nd $4f$-spin dynamics at low energies $(\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\lesssim}1 \mathrm{meV}).$ In all samples we find at high temperatures a quasielastic line (Lorentzian) with a linewidth which decreases on lowering the temperature. The temperature dependence of the quasielastic linewidth $\ensuremath{\Gamma}/2(T)$ can be explained with an Orbach process, i.e., a relaxation via the coupling between crystal field excitations and phonons. At low temperatures the $\mathrm{Nd}\ensuremath{-}4f$ magnetic response $S(\mathbf{Q},\ensuremath{\omega})$ correlates with the electronic properties of the ${\mathrm{CuO}}_{2}$ layers. In the insulator ${\mathrm{La}}_{2\ensuremath{-}y}{\mathrm{Nd}}_{y}{\mathrm{CuO}}_{4} (y=0.1,0.3)$ the quasielastic line vanishes below 80 K and an inelastic excitation occurs. This directly indicates the splitting of the ${\mathrm{Nd}}^{3+}$ ground state Kramers doublet due to the static antiferromagnetic order of the Cu moments. In ${\mathrm{La}}_{1.7\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nd}}_{0.3}{\mathrm{CuO}}_{4}$ with $x=0.12,0.15$ and ${\mathrm{La}}_{1.4\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Nd}}_{0.6}{\mathrm{CuO}}_{4}$ with $x=0.1,0.12,0.15,0.18$ superconductivity is strongly suppressed. In these compounds we observe a temperature independent broad quasielastic line of Gaussian shape below $T\ensuremath{\approx}30 \mathrm{K}.$ This suggests a distribution of various internal fields on different Nd sites and is interpreted in the frame of the stripe model. In ${\mathrm{La}}_{1.8\ensuremath{-}y}{\mathrm{Sr}}_{0.2}{\mathrm{Nd}}_{y}{\mathrm{CuO}}_{4} (y=0.3,0.6)$ such a quasielastic broadening is not observed even at lowest temperature.

We have measured phason hopping in isotopic powder samples of decagonal Al-Co-Ni quasicrystals, by time-of-flight quasielastic neutron scattering. The three isotopic samples were based on ${}^{\mathrm{nat}}\mathrm{Ni},$ and 99.5% enriched ${}^{58}\mathrm{Ni}$ and ${}^{60}\mathrm{Ni}.$ Two characteristic jump times have been observed. The anomalous temperature dependence observed in the icosahedral phases i-Al-Cu-Fe and Al-Mn-Pd is confirmed.

We use a recently developed method based on inelastic nuclear-resonant absorption of x rays to measure the iron-partial vibrational density of states (VDOS) in a quasicrystal. The results differ from those obtained by inelastic neutron scattering in an astounding way: whereas the neutron results exhibit a very smooth and featureless behavior, the iron-partial VDOS is strongly peaked at one energy value.

We performed inelastic magnetic neutron scattering experiments on the high-Tc superconductor La2−x−ySrxREyCuO4 with RE=Dy (x=0, 0.25 and y=0.09) and RE=Nd (x=0, 0.12, 0.15, 0.2 and y=0.1, 0.3). In this system an additional low temperature phase transition occurs, which has a strong impact on many physical properties. In particular the suppression of superconductivity around the Sr-concentration of x=1/8 is of interest. We investigated the magnetic behavior of the RE-moments in order to obtain information about the Cu magnetism in the CuO2-layers. The magnetic response shows qualitative differences at low temperatures which correlate with the electronic properties of the samples. The results are discussed with respect to the static stripe model of spins and holes.

Experimental results obtained by coherent neutron scattering with a time-of-flight spectrometer on the nonfragile glass-forming liquid ${\mathrm{Na}}_{0.5}{\mathrm{Li}}_{0.5}{\mathrm{PO}}_{3}$ (${T}_{g}=515\mathrm{K},$ ${T}_{m}=749\mathrm{K}$) in a wide temperature range (300\char21{}773 K) are presented. As predicted by the mode coupling theory (MCT), the resulting dynamical structure factor has a $q$-independent form in the intermediate ${\ensuremath{\beta}}_{\mathrm{fast}}$ regime. Furthermore, it is also found that the $q$ dependence of the dynamical structure factor is the same for the boson peak and the quasielastic region. Thus, a phenomenological model assuming that the quasielastic component is related to the damping of the localized vibrational modes that give rise to the boson peak is discussed. Though the spectra are well described over a wide temperature range, it is shown that the temperature ${T}^{*}$ where the boson peak becomes overdamped is significantly above the melting temperature in this system and thus cannot be associated to any onset of metastability as previously suggested. On the other hand, a qualitative discussion of the susceptibility spectra above ${T}_{g}$ in the frame of the MCT leads to a possible crossover temperature ${T}_{c}$ near 620 K $(\ensuremath{\sim}1.2{T}_{g})$ close to the temperature of the decoupling phenomenon of the relaxation time scales observed by ${}^{31}\mathrm{P}$ NMR in this system. This ${T}_{c}$ value is also compatible with a power-law temperature dependence of the structural relaxation time scale deduced from the viscosity data.

Surface melting of the (111) face of methane films adsorbed on graphite is investigated by quasielastic neutron scattering with special attention paid to translational diffusion within the liquid-like layer parallel and normal to the surface. The comparison of the experimental spectra measured in different geometries clearly shows diffusion anisotropy. Simulations of the data provide an estimation of the anisotropy as a function of temperature.

We report the first time-of-flight (T.O.F.) quasielastic neutron scattering results on hydrogen motion in quasicrystals. The data show that some of the hydrogen is chemically bound.

4 LURE (CEA-CNRS), UniversiteParis-Sud, Orsay, France ABSTRACT Quasielastic neutron scatter- ing experiments performed on yeast phospho- glycerate kinase in the native form and dena- tured in 1.5 M guanidinium chloride reveal a change in the fast (picosecond time scale) diffu- sive internal dynamics of the protein. The momentum and energy transfer dependences of the scattering for both states arefitted by an analytical model in which, on the experimen- tally accessible picosecond time scale and ang- strom length scale, the dynamics of a fraction of the nonexchangeable hydrogens in the pro- tein is described as a superposition of vibra- tions with uniform diffusion in a sphere, the rest of the hydrogens undergoing only vibra- tional motion. The fraction diffusing changes, from D60% in the native protein to D82% in the denatured protein. The radius of the sphere also changes slightly, from D1.8 A in the native protein to D2.2 A in the denatured protein. Possible implications of these results for the general protein folding problem are discussed. Proteins 28:380-387, 1997 r 1997 Wiley-Liss, Inc.

We present a simple model that allows to understand the essential features behind the mechanisms of assistance to phason hopping. We provide a discussion on the length scales in the phason dynamics and the relation between phason dynamics and phason elasticity. We propose an alternative explanation for the diffuse scattering observed in icosahedral quasicrystals that resembles more the explanations offered in decagonal systems, putting the phenomena on a footing of universality, and showing that the currently adopted interpretation for icosahedral systems is not exclusive.

We present time-of-flight quasielastic neutron scattering data with isotopic contrast on phason hopping in perfect icosahedral ${\mathrm{Al}}_{62}{\mathrm{Cu}}_{25.5}{\mathrm{Fe}}_{12.5}$ quasicrystal powder samples. In total one Fe jump on the time scale $\ensuremath{\Elzxh}/2.0\ifmmode\pm\else\textpm\fi{}0.5\ensuremath{\mu}$eV, two Cu jumps ( $\ensuremath{\Elzxh}/55$ and $\ensuremath{\Elzxh}/253\ensuremath{\mu}$eV), and a collective jump phenomenon [ $\ensuremath{\Elzxh}/(900\ifmmode\pm\else\textpm\fi{}200\ensuremath{\mu}$eV)] involving all atomic species have now been identified. We also explain the unusual temperature dependence of the phason dynamics that has been universally observed in the past.

A high-resolution inelastic neutron scattering experiment has been carried out in the \ensuremath{\alpha} and \ensuremath{\mathrm{B}} low-temperature phases of solid oxygen. Direct comparison between the spectra of both phases as well as with previous experiments and lattice dynamics calculations, allow one to identify a feature appearing at \ensuremath{\approx} 4.3 meV in the low Q region of the spectra as arising from the antiferromagnetic excitations, the existence of which had been postulated from optical work.

Ammonia adsorbed on a MgO{100} surface represents a model system of a molecule in an environment of incompatible symmetry. High-resolution neutron spectroscopy was used to measure the rotational spectra of ammonia molecules at nine different coverages in the range . The energies of the four lowest rotational levels are derived and analysed within the rotation - translation-coupling model. The situation with isolated molecules is realized at very low concentrations. With increasing coverage, intermolecular interaction leads to an increase of the strength of the rotational potentials, but ammonia molecules do not order orientationally below . The centre-of-mass eccentricity stays constant. At the highest coverages , intermolecular interaction dominates and suppresses tunnelling. A model of coexisting phases is presented to explain simultaneously the sharpness of the tunnelling lines and the variation of their energy and intensity with coverage.

The first (preliminary) results on the anisotropy of the quasi-elastic neutron scattering signal from atomic hopping (phasons) in quasicrystals are presented. The results taken with a triple-axis spectrometer at various Q -vectors with a modulus of 2.85 A −1 in a plane perpendicular to a twofold axis show that the intensity is less along the fivefold axis. This suggests that the jumps observed occur along a twofold or a threefold axis.

Rotational tunneling of CH 4 molecules in microporous and mesoporous silicate structures has been studied by inelastic neutron scattering. The rotational excitation energies of the spherical quantum rotor CH 4 are a sensitive probe of the strength and symmetry of local potentials. Measurements of methane molecules isolated in micropores of dodecasil 3C and of solid methane condensates in mesoporous silicagel and in the mesoporous channels of MCM-41 show broad distributions of rotational energies. A simple model which is based on a distribution of local potentials was applied to the data. In dodecasil 3C time-resolved measurements did not show spin conversion behaviour in a measuring time of 15 h. Even in pores with diameters as large as 100 A no tendency towards the ordering of the CH 4 phase II was found.

High-resolution neutron spectroscopy was used to measure the rotational spectra of isolated NH 3 molecules adsorbed on MgO{100} surfaces. The energies of the four lowest rotational levels were analyzed first by the single-particle model and secondly by a model of coupled rotation of the molecule and its centre of mass. Only the latter potential surface can be reproduced with reasonable accuracy on the basis of semi-empirical pair interaction potentials. Thus this system is among the first examples of coupled internal and off-centre rotation in the quantum regime. The off-centre site of the NH 3 and the coupled motion are confirmed by a recent ab initio calculation.

We illustrate a recently described method to calculate total quasi-elastic scattering functions on some realistic problems. We also show how this method reaches its full power by combining it with group representation theory.

Three remarkable anomalies related to the acoustic branches of quantum paraelectric (QPE) oxides are presented. In SrTiO 3 , an additional shear-like sheet is observed for modes propagating in the tetragonal plane, and shear branches polarized parallel to the tetragonal axis show an anomalous softening on cooling in the QPE-regime. Both in SrTiO 3 and KTaO 3 , a new doublet is observed in Brillouin scattering which is due to second sound, the wave-like propagation of heat. These effects are discussed in terms of large anharmonic polarization clusters.

We report the first very low temperature (from 93 mK to 7.3 K) specific heat data of a modulated incommensurate dielectric (Cl${\mathrm{C}}_{6}$${\mathrm{H}}_{4}$${)}_{2}$S${\mathrm{O}}_{2}$. They reveal three different contributions in excess of that of normal acoustic phonons: low frequency excitations predominant below 0.2 K, a phason branch above 0.7 K, and optical phonon and amplitudon branches around 4.5 K. This analysis is done considering neutron scattering results also.

We present here new results on phason dynamics (atomic hopping) in quasi-crystals. An iron jump has been observed by spectroscopy Mossbauer in the system AlFeCu. Its relaxation time is very different from the one of the 3.9 A Cu jump previously determined by neutron scattering. Two distinct atomic jump times have also been observed by neutron scattering in the system AlMnPd. They exhibit the same unusual temperature dependence as the Cu one, such that it seems to be universal.

In the context of structural investigations concerning the function of the membrane protein bacteriorhodopsin (BR) as part of purple membrane (PM) we have investigated the diffusive motion occurring in the PM. In order to analyse these motions on the molecular level, a study using quasielastic incoherent neutron scattering (QINS) was performed with oriented PM stacks. Experiments with samples, hydrated in D2O, were carried out under dark conditions (without pump activity) at room temperature. The spectra obtained with two energy resolutions (50 and 16 μeV (FWHM)) were used for fits with a model consisting of five components. The treatment follows the idea of describing one part of the scattering intensity with a certain number of motions by theoretical terms which are already known from other studies, and representing the remaining scattering intensity, corresponding to as yet unknown types of motion, by a general two-site jump model. This procedure allows one to characterize the predominant components, the three-site jump motions of protons in methyl groups and the general two-site ‘jumps’ of protons in polypeptide side groups, by correlation times, jump distances and statistical weights. The jump distances obtained from the two-site model are similar to the spatial extent of the side groups (dts = 2–4 A); the correlation times show, that rotational diffusion of protons in methyl groups (τmet = 4.3 ps) is a much faster process than reorientation of the larger side groups (τts = 19 – 110 ps).

We give a simple deductive derivation of the Dirac equation for a free particle. Our construction provides a clear distinction between what are the physical contents and what are purely mathematical expedients in the formalism of quantum mechanics (QM).

Incoherent quasi-elastic neutron scattering is applied to study the local diffusion and chain dynamics of L-alpha-diplamiotylphosphatidylcholine molecules in oriented model membranes. Different motions are distinguished by changing the hydration of the multilayers as well as by measuring below and above the gel-to-liquid crystalline phase transition. The time range of the utilized time-of-flight spectrometer permits to observe two types of motion to be observed more closely: chain defect motions and the local diffusion of the whole molecule in its solvation cage. Oriented lipid membranes are a useful system for the observation of chain defects, as they can be macroscopically oriented, in contrast to most polymers. As a representative model for a chain defect a kink is chosen and the corresponding scattering functions are derived. The kink motion can explain the entire dynamics seen in the gel phase, and the lifetime of such a defect was found to be 10–15 ps, in good agreement with theoretical predictions. On the other hand the dynamics in the liquid crystalline phase cannot be explained even by a superposition of several kinks and thus requires the consideration of an additional motion: the local diffusion of the molecule in its solvation cage. The size of the solvation cage is increasing with multilayer hydration and reduced temperature. Particularly interesting in view of recent discussions about the origin of the short-range repulsive forces between membranes is the experimental finding of an out-of-plane motion with an amplitude of 1–1.5 A, which cannot be explained by the undulation of the whole membrane.

It is a well-known fact that fivefold rotational symmetry is incompatible with crystalline lattice periodicity. Suppose that a Bravais lattice would contain a regular pentagon (Figure l), then it is possible to construct a smaller pentagon that also belongs to the lattice, as can be seen in Figure 1. This implies that a periodic lattice with fivefold symmetry can not satisfy the Delaunay condition. Thus, it came as a big surprise when Shechtman et al. (1) discovered quasicrystals (QC) (2) that exhibited diffraction patterns with five-fold (icosahedral) symmetry and sharp Bragg peaks characteristic of long-range order. Later, QC with eight-fold, tenfold and twelve-fold 2D symmetry were reported. The origin of this apparent paradox lies in a confusion between the concepts of long-range order and periodicity. Quasiperiodic lattices or tilings, such as illustrated in Figure 2 (for eightfold 2D symmetry), are not periodic but do give rise to diffraction patterns with sharp Bragg peaks due to their und...

We present a theorem that in coherent quasielastic neutron scattering on a confined and limited system of identical particles only the configuration-configuration correlations have to be examined without any hindsight on the distribution of the particles inside the configuration. This constitutes a strong simplification with respect to the problem where all particle-particle correlations would have to be accounted for. The method is illustrated on an example of correlated hopping on a piece of an octagonal quasiperiodic tiling. With some modifications the method can also be used for incoherent scattering. A further simplification of the calculations both for coherent and incoherent scattering functions is obtained by introducing a new matrix notation that allows fully for the symmetry properties of the problem

Ammonia molecules adsorbed as 0.6 monolayers on MgO (100) surfaces were investigated by neutron spectroscopy. The observed lines at 0.47, 1.59 and 2.04 meV can be interpreted in terms of hindered one-dimensional rotation around the molecular symmetry axis in a potential of higher than threefold symmetry. This is the first observation of rotational tunnelling of a polar molecule bound to an ionic surface. Model calculations on the basis of atom—atom potentials show that the rotational potential is not due to interactions with the substrate.

Barium-nitrade-hydride is one of the few documented H--ionic conductors. It crystallises in the [alpha]-NaFeO2-structure (space group R m) with distorted Ba-octahedra centred alternately by H- and N3-. Neutron diffraction shows that the hydrogen ions are distributed independent of temperature over lattice sites: 85% on (3a) and 15% on (6c). At T > 525 K a sharp increase of the ionic conductivity can be observed. The diffusion process takes place by uncorrelated jumps of hydrogen ions between regular lattice sites. Using quasielastic neutron scattering techniques and a planar jump diffusion model, we were able to determine the mean residence time [tau] on a H--site to be [tau]=20±2ps and the diffusion constant D to be 2.1±0.3×10-5 cm2/ /s at T=823 K.

Inelastic neutron scattering has been used to study molecular tunnelling in the p-tert-butylcalix[4]arene(2:1)p-xylene complex. At low temperature the measured spectra show several bands between 0·63 and 2·6 meV which are interpreted as being due to transitions between tunnel-split librational states of the p-xylene methyl groups. Effects of coupling between a CH3 group and the whole p-xylene molecule have been observed. The rotational barrier provided by the host lattice is very small and the methyl groups behave as almost free quantum rotors.

Combined Raman, Brillouin and ultrasonic measurements have recently shown the existence, in salol, of a dynamical transition near 260 K. This transition is marked by a weak cusp in the non-ergodicity parameter, ƒ0(T), of the mode coupling theory. In parallel, we have also performed a neutron study of the Mibemol time-of-flight spectrometer at the LLB (Saclay). Measurements of the quasi-elastic scattering are presented which also reflect the occurrence of a transition, revealing a cusp in the same temperature range as the Raman and Brillouin results, thus providing further confirmation of the mode coupling theory.

The vibrational spectra of Ni(NH 3 ) 2 Ni(CN) 4 ·2C 6 D 6 are consistent with distorted NH 3 groups despite the presence of quantum free rotation of these groups even at 1.8 K. The calculated INS vibrational spectrum of an isolated NH 3 NiNH 3 unit is in good agreement with the observed spectrum and reveals a pronounced anisotropic effect of the Debye-Waller factor due to NH 3 rotations. Possible origins of the splitting of the molecular E-modes of the NH 3 groups are investigated and it is probable that the effective symmetry of the local environment is Cs. There is evidence of vibrational/rotational transitions.

We have studied a hopping process in perfect icosahedral Al62Cu25.5Fe12.5 powder samples by time-of-flight (TOF) quasi-elastic neutron scattering. Within the (ω, Q)-window of the present experiment the signal observed is entirely due to copper motion. Its dependence on the neutron momentum transfer Q suggests that this process remains confined in space and that it has a characteristic length of (3.9 ± 0.25) A. Its dependence on the temperature T is interpreted as giving evidence for a depinning energy of 0.75 eV.

We describe recent time-of-flight neutron scattering measurements of the quasielastic scattering in icosahedral AlCuFe. The broadening of the quasielastic component at high temperatures may be accounted for by atomic hopping on a time scale of about 7 ps. The relationship between this hopping and phason modes in quasicrystals is discussed.

New inelastic neutron-scattering and Brillouin-scattering results are reported for pressure-oriented SrTiO, crystals at temperatures below the structural transition. The measurements reveal an unexpectedly large number of anomalies at small frequencies and wave vectors in the quantum paraelectric region. The anomalies include a strong depression of a TA-branch at unusually small wave vectors, an apparent splitting of the same branch, the lifting of a selection rule for the structural mode, and an anomalous temperature dependence of the elastic constant C,, .

The cyclopalladation reaction of the liquid crystalline species p-dodecyloxy-p'-methylazobenzene (DOMAB) leads to the bimetallic metallo-mesogene [(DOMAB)Pd Cl]2. Both the organic compound and its palladium organometallic complex have been investigated by quasielastic neutron scattering experiments in the nematic and smectic A phases. For DOMAB, the results exhibit a continuous rotation as expected for usual rod-like liquid crystals, while for [(DOMAB)Pd Cl]2 they suggest a model wherein only some aliphatic protons can give a rotational diffusion.

Quasi-elastic neutron scattering of H2O-saturated powders of graphitized carbon black and talc show unfrozen water persisting below the bulk freezing point, to temperatures below -30°C. Several features are consistent with surface melting at the ice-substrate interfaces, but the temperature dependence disagrees with the standard theory. The discrepancies are attributed to size effects, and are supported by model calculations. Similarities with the behavior of unfrozen water in subzero soils suggest that they share the same fundamental mechanisms.

Brillouin and neutron-scattering measurements of TA-phonon branches are reported for SrTiO3 at small wave vectors and at temperatures well below the 105 K structural transition. Anomalous softening in the former measurements, and dispersion in the latter, are found in the quantum paraelectric regime. Furthermore, an unexpected loss of scattered intensity is observed on the TA[100]-phonon branch at temperatures centred around 30 to 40 K.

By means of time-of-flight inelastic neutron scattering we have studied two samples with nominal compositions Al62Cu25.5Fe12.5 (perfect sample) and Al63Cu24.5Fe12.5 (imperfect sample). The former is a perfect quasi-crystal with sharp Bragg peaks over the whole temperature range from room temperature up to 800 °C. The latter exhibits morphological evolution to a low-temperature imperfect quasi-periodic state starting around 650 °C. Spectra were recorded at 790 °C, 750 °C, 675 °C, 625 °C and room temperature with an incident wavelength of 7 A (i.e. 77 μeV elastic resolution). In both samples at high temperature a quasi-elastic component (a Lorentzian of width Γ = 95 μeV) is observed which disappears around 650 °C and is attributed tentatively to phason hopping. At 790 °C the crystalline β-phase Al50Cu25Fe25 does not show such a quasi-elastic component. We clarify different concepts of phason existing in the literature.

We propose a monoatomic model for a quasicrystal transition as observed recently in systems with icosahedral [3] and decagonal [5] symmetry. It is developed here for the case of decagonal symmetry and is inspired by the experimental results on the system Al-Cu-Co-Si [5,6]. The model goes beyond the purely geometrical description by an important physical aspect: the transition mediates through a single atomic jump distance such that only one unique double-well potential has to be invoked to describe it; in conformity with the symmetry there are 10 jump vectors. In the framework of the model, the microcrystalline state is energetically more favourable than a monocrystalline approximant phase.

We have studied the dynamics of the iron spins in a 6.5% iron doped Bi 2 CaSr 2 Cu 2 O 8 superconducting compound with T c =46.5 K by time of flight (TOF) neutron scattering experiments. The quasielastic and elastic intensities and the width of the quasielastic signal strongly vary with temperature. At low temperature, a slowing down of the iron spin dynamics is observed, starting around 60 K, i.e. above T c . At higher temperature, an unusual temperature dependence of the quasielastic intensity is observed in the doped Bi compound as previously described in Fe doped YBa 2 Cu 3 O 7 .

We have performed molecular-dynamics simulations of displacement cascades in copper in order to investigate the non-equilibrium ultra-short time damage and to evaluate the possibility of observing it experimentally in-situ (e.g. in a pump/probe laser experiment). The atomic trajectories have been analysed by calculating their X-ray diffraction patterns as a function of time. The results show that an integrated X-ray intensity can indeed be used to evidence the irradiation effects. Even though the number of Frenkel defects is large, the main effect of the irradiation showing up in the X-ray intensities at ultra-short times is an important alteration of the lattice vibrations. On the basis of these results, a new pump/probe set-up is proposed.

In the inelastic neutron scattering spectra of Ni-Ni-Bz(d6) obtained at 1.8 K the authors observed transitions differing only slightly, but systematically, from those between uniaxial free rotator levels of ammonia groups. They interpreted the deviations as the influence of a weak hindering potential. A twelve-fold cosine potential with a barrier height V12 of 16+or-3 meV together with a rotational constant of 0.71+or-0.02 meV for NH3 gave a satisfactory description of the experimental results. Spectra obtained at higher temperatures (up to 51 K) showed qualitatively the transition from quantum to classical behaviour.